Chromatograph mass spectrometers which consist of the combination of a chromatograph, such as a gas chromatograph (GC) or liquid chromatograph (LC), and a mass analyzer, such as a quadrupole mass analyzer, have been widely used for qualitative and quantitative determinations of various components contained in a sample. In general, when the quantitative determination of a known compound is performed with a chromatograph mass spectrometer, an SIM measurement method for selectively and repeatedly detecting one or more ions having previously specified mass-to-charge ratios is used.
When the quantitative determination of a known compound is performed using a chromatograph mass spectrometer consisting of a chromatograph (e.g. GC or LC) coupled with a triple quadrupole mass analyzer, an MRM measurement method is used, in which an ion having a specific mass-to-charge ratio (precursor ion) is selected by a front quadrupole mass filter, this ion is then fragmented in a collision cell by a collision-induced dissociation process, an ion having a specific mass-to-charge ratio among the product ions produced by the fragmentation is selected by a rear quadrupole mass filter, and the selected ion is detected. The MRM measurement method is advantageous in that the influence of foreign substances can be removed by the two quadrupole mass filters, so that the S/N ratio of the detection signal is improved and a higher level of sensitivity is achieved in quantitative determinations.
Normally, when an SIM or MRM measurement is performed with a chromatograph mass spectrometer in this manner, the component to be detected is previously known and the required task is to detect that component with the highest possible level of sensitivity. To this end, the analysis operator must set appropriate analysis conditions so that the highest possible level of sensitivity of the analysis will be achieved. A procedure for setting the analysis conditions in a conventional and common type of chromatograph mass spectrometer is described using FIGS. 7-9C.
When an analysis operator performs a predetermined operation on a control computer, a method-editing window 500 as shown in FIG. 7 is displayed. In the present example, the method-editing window 500 has an event information table 501 in its upper area and a channel information table 502 in its lower area, the latter table allowing the setting of the conditions for SIM-measurement-type events. An “event” is a measurement to be performed under one analysis condition in a series of analyses. In the event information table 501, each row corresponds to one “event”, while the columns in table 501 show various items of information related to each event, such as the event number, analysis mode (labelled as “TYPE”), ion polarity (labelled as “+/−”), mass-to-charge ratios of the ions to be monitored, and measurement time range. The “SIM-measurement type” is a type of measurement for selectively detecting an ion having a specific mass-to-charge ratio. Specifically, it includes the SIM measurement and the MRM measurement.
On the method-editing window 500, the analysis operator clicks one of the two radio buttons arranged in the polarity selection button area 503 to select the polarity of the ion to be analyzed in an event which is to be added. Subsequently, the analysis operator clicks one of the buttons arranged in the analysis mode addition button area 504 to select an analysis mode to be added (e.g. “MRM”, “Precursor Scan”, etc.). By such operations, an event for performing the selected analysis mode is added to the event information table 501. In the example of FIG. 7, three events have been set, with the MRM measurement selected as the analysis mode in all of them.
In the case where the analysis mode is the SIM-measurement type, or more specifically, in the case of an MRM or SIM measurement, a plurality of ions can be set in the channel information table 502 as the ions to be monitored in one event. Furthermore, in the case of the MRM measurement, the mass-to-charge ratio of the precursor ion and that of the product ion are individually set, as shown in FIG. 7. The two text boxes arranged in the measurement time input area 505 allows the setting of the measurement time range of the event by entering the measurement starting time and measurement finishing time. The measurement time range set in this area is graphically shown by a bar graph in the measurement-time display field 501a in the event information table 501 (see Patent Literature 1).
In general, analysis operators need to pay attention to the following points in setting the measurement time range of each event:
(1) The measurement time range should be set with a certain amount of extra time before and after the retention time of the target compound, since the point in time at which the compound is actually eluted from the column does not exactly coincide with its retention time.
(2) In the SIM-type measurement, the overlapping of a plurality of events should be avoided as much as possible in order to maximize the detection sensitivity. Naturally, in the case of a simultaneous multicomponent analysis, it is impossible to completely avoid the overlapping of the events. Accordingly, the overlapping of the events is allowed for a compound for which the problem of detection sensitivity is unlikely to occur (e.g. when the content of the compound is known to be high), whereas care should be taken to minimize the overlapping of the events for a compound for which the problem of detection sensitivity is likely to occur (e.g. when the content of the compound is known to be low).
After setting the events and appropriately adjusting the measurement time range of each event, when the analysis operator clicks a loop-time display button 506 in the method-editing window 500, a loop-time checking window 600 as shown in FIG. 8 is displayed. The relationship between the event time and the loop time is hereinafter described with reference to FIGS. 9A-9C.
FIGS. 9A-9C are model diagrams showing the relationship between the event time and the loop time in the case where a plurality of events are temporally overlapped. In this example, as shown in FIG. 9A, “Event 1(+)” (where “+” denotes a mode for detecting positive ions, while “−”, which will be mentioned later, means a mode for detecting negative ions) includes four channels labelled as Ch1-Ch4 and sequentially detects four kinds of ions with different mass-to-charge ratios in a time-shared manner. In the case of a quadrupole mass analyzer, each of the ions with different mass-to-charge ratios is selected by switching the voltage applied to the quadrupole mass filter. Therefore, every time the channel is switched within one event, a “pause time” in which the collection of data is suspended is set. During this pause time, the applied voltage is switched and stabilized. After that, the period of time in which the detector actually receives and accumulates ions, i.e. the data-collecting time, is provided as the “dwell time.”
In the present example, as shown in FIG. 9B, the four events, i.e. Event 1(+), Event 2(+), Event 3(−) and Event 4(−) are temporally overlapped. These four events are sequentially performed in a time-shared manner. The period of time required for one cycle of processes in which each of these four events is performed one time is the loop time. For example, in Ch1 of Event 1, in which a certain kind of ion species is detected, the detecting operation is performed in such a manner that the next detection of this ion species is performed after the loop time has elapsed since the previous detection of the same ion species. In other words, the interval of time of the detection of the same ion species is the loop time. As can be understood in FIG. 9C, when observing a peak on a chromatogram, using a longer loop time increases the interval of the neighboring data points, making it difficult to correctly grasp the peak shape. Therefore, particularly in the case of a quantitative analysis, it is important to reduce the loop time so that it does not exceed a certain value.
When an ion having a different polarity is to be detected, it is necessary to change the polarity of most of the voltages applied to the ion source, ion transport optical systems and other components in the mass analyzer. Therefore, as shown in FIG. 9B, when the polarity of the ion to be detected changes, a polarity-switching time is provided before the event time.
As shown in FIG. 8, an automatically calculated loop time is displayed in the loop-time listing table 601 arranged in the loop-time checking window 600. The automatic calculation of the loop time is disclosed in Patent Literatures 1 and 2 as well as other documents.
As can be understood from the foregoing explanations, the loop time normally depends on the number of overlapping events. Accordingly, the loop time is calculated for each range of time in which the number of overlapping events changes. In the largest loop-time display field 602 below the loop-time listing table 601, the value of the largest loop time within the entire measurement time is displayed. While visually checking the loop time in this window 600, the analysis operator appropriately adjusts the dwell time, event time, measurement time range and other parameters set in the method-editing window 500 so that the number of data points per one peak on the chromatogram will be an appropriate value.
As explained earlier, the dwell time is the period of time in which the acquisition of the data based on the ion intensity signal is actually performed. Accordingly, it considerably affects the detection sensitivity. Therefore, normally, a long dwell time is set when the detection sensitivity is low, while a short dwell time is set when the detection sensitivity is high. However, setting too short a dwell time lowers the level of the ion intensity signal and causes a decrease in the S/N ratio or worsens the peak shape on the chromatogram, which consequently decreases the accuracy of the peak area and possibly lowers the reliability of the quantitative determination. On the other hand, setting too long a dwell time causes a corresponding increase in the loop time, which also lowers the reliability of the quantitative determination due to various problems, such as the incorrect grasping of the peak top on the chromatogram or an incorrect approximation of the shape of the curve in the rising or falling phase of the peak. For these reasons, it is not always easy to appropriately set the dwell time; even an analysis operator with a certain amount of experience normally needs a considerable amount of time for this task.